The present invention relates to bipolar transistors and, more particularly, to enhancing the alpha of a bipolar transistor.
The alpha (a) of the transistor is defined to be the ratio of its collector current to its emitter current (I.sub.C /I.sub.E). Ideally, we would like a to be unity, but due to the base current, I.sub.E is larger than I.sub.C and, thus, a is less than unity. Furthermore, it is common to sense or measure the current in the emitter leg of a transistor and have the collector coupled to additional circuitry for transmitting this measured current. This is typically done since the circuitry coupled to the collector of a transistor may be high voltage while the circuitry coupled to the emitter is usually low voltage. Therefore, any difference in the emitter and collector currents of a transistor can present a problem when precision between the two is required.
One method of enhancing the alpha of a transistor is well known as the Darlington method which involves taking the base current of a transistor to be alpha enhanced, and passing it through a driver transistor wherein their collectors are tied together. The objective of the Darlington method is to effectively add in the base current of the transistor to be enhanced to its collector current so that the collector and emitter currents are equal thereby making the alpha of the transistor to be enhanced equal to unity. However, a problem with the Darlington method is that since both transistors have their collectors tied together, if the transistor to be enhanced must be selected for high voltage, then so must the driver transistor and, thus, requiring two transistors with high breakdown voltages. The problem is further increased when the alpha enhancement circuitry, in this case the driver transistor, is desired to be incorporated on an integrated circuit which is used to drive an external transistor that is to be alpha enhanced. Since the collectors of the external and driver transistors are tied together, the enhancement circuitry on the integrated circuit must also be designed for high voltage which will require much die area on an integrated circuit.
Hence, a need exists for an improved circuit to enhance the alpha of a transistor that is independent of the signal at the collector of the transistor to be enhanced so that the circuit can be designed for low voltage signals.